Di(2-ethylhexyl)phthalate (DEHP) and phenobarbital (PB) were studied in model systems for initiation and promotion in skin, mouse and rat liver, and JB6 mouse epidermal cells. DEHP was found to be a liver tumor promoter, a second stage skin tumor promoter, a "weak" complete skin tumor promoter and a promoter of JB6 cells to anchorage independence in mice. It promoted skin tumors even only after thirty days of dietary exposure for the mouse. PB was also a tumor promoter in mouse liver . DEHP, however, was not a tumor promoter in rat liver despite its similar hepatotoxicity in rats and mice.